Reproduction processes are known wherein positive-working photopolymerizable elements and negative-working photosensitive elements are exposed to actinic radiation through a transparency to produce a pattern of tacky and complementary nontacky image areas. A positive-working photopolymerizable element is described, for example, in Chu and Cohen, U.S. Pat. No. 3,649,268 and a negative-working photosensitive element is described in Cohen and Fan, U.S. Pat. Nos. 4,174,216 and 4,191,572. The image may be developed by toning with a suitable particulate material which desirably adheres only in the tacky image areas. Excess toner which may be present is removed from the nonimage areas leaving, for example, an image which is a proof of the original or which can be transferred to another surface. Multilayer multicolor proofs such as surprint or overlay pre-press proofs are a particularly advantageous use of such toned elements. The process for preparation of such proofs is well known in the graphic arts, and is described in detail, for example, in U.S. Pat. Nos. 3,649,268 and 4,174,216, the teaching of which is hereby incorporated by reference.
In view of the importance of pre-press proofing to the printing industry and the problems inherent therein, improved toners are of significant commercial importance. For acceptable toning of photosensitive elements having imagewise tacky and nontacky areas, it is important that the toner adhere evenly and reproducibly to the tacky image areas while being easily and substantially completely removable from the corresponding nonimage areas. Some recognized improvements in nonelectroscopic toners are those described in Chu and Manger, U.S. Pat. No. 3,620,726, mixtures using these toners described in Gray, U.S. Pat. No. 3,909,282, and the toners of Manger, Fickes and Long described in U.S. Pat. No. 4,215,193. Improved toner applicators are the subject of Sandner, U.S. Pat. No. 4,019,821 (hand operated toning) and Tobias, U.S. Pat. No. 4,069,791 (automatic toning).
Application of toners to tacky image areas of photosensitive elements has the problem that the toners tend to produce background color in the nontacky areas as well. This color is frequently very difficult to remove completely. Generally an attempt is made to remove the excess toner from the nontacky areas, e.g., by air impingement or by wiping with a cloth, brush, or other removal means. While wiping is a simple operation, it is aesthetically unpleasant, requires operator involvement, takes time and uses a consumable (e.g., a Las-Stik.RTM.) cloth.
Wiping may also exacerbate the difficulties of toner removal by generating static electricity. It is known that static is generated by rubbing dissimilar materials together. The amount of static generated varies, depending on the nature of the particular photosensitive layer being toned and the toner removal means. For example, cleaning brushes close to the photosensitive layer in the triboelectric table would be expected to generate a relatively small charge. Thus it would be expected that an acrylic brush would generate only a small charge with respect to an element that contains an acrylic compound as a major component of the photosensitive layer. The toner, however, can also generate a charge against the brush or other cleaning means and the photosensitive element. If the toner, cleaning means, and element were to have an identical place in the triboelectric table, substantially no static charge would be generated. Such a system, while desirable in this respect, is generally not realizable. Despite the problems, it is nevertheless usually desirable to reduce background color as much as possible by removing excess toner. Even after manual or machine cleaning operations, however, some undesirable background color is usually present in the nontacky areas.
The term "background color", as used herein, is the color present in the nontacky background areas of an exposed and toned positive-working or negative-working photosensitive element prior to any step being taken to remove toner therefrom. Background color is the sum of two components. "Stain" is used herein to mean the color which is normally not capable of being removed from the background areas despite thorough wiping. "Clean-up" is the color normally capable of being removed from the background areas, e.g., by wiping, air impingement, etc.
It has been found that a given quantity of toner is used more efficiently and lasts longer when applied by machine than by hand. Automatic toning machines have also improved the uniformity and consistency of short-term proof-to-proof toning density, so that changes in toner performance can be more readily assessed. These factors increase the need for toners which perform uniformly and reproducibly over time. The need for consistent toning despite toner aging is exacerbated by the international nature of the graphic arts business and the length of time sometimes needed to get toners from the manufacture to the ultimate user.
U.S. Pat. No. 4,397,941 to Fickes teaches dry nonelectroscopic toners comprising pigmented organic resin particles having a size distribution of 0.2 to 30 micrometers, which are surface treated with at least 0.5% by weight of a slip agent selected from: a silicone oil having a weight average molecular weight of about 230 to 50,000; a saturated hydrocarbon having a weight average molecular weight of about 200 to 10,000; or a fluorocarbon compound having a weight average molecular weight of about 500 to 500,000, in combination with at least 1% by weight of a defined antistatic agent. Suitable antistatic agents include anionic, cationic, amphoteric and nonionic compounds. Such toners exhibit good clean-up and are substantially nonstaining in the nontacky areas of the photosensitive elements used to prepare multilayer pre-press proofs. On the other hand, the optical density of toning with some such toners, e.g., magenta and cyan, tends to be below the density required for pre-press proofs. In addition, the consistency of toning density from proof to proof deteriorates as the toner ages.
U.S. Pat. No. 4,546,072 to Matrick describes dry, nonelectroscopic toner particles which are surface coated with a liquid slip agent, such as silicone oil or a fluorocarbon compound, and a solution containing a cationic polymeric quaternary ammonium antistatic agent. These toners exhibit clean-up qualities and nonstaining of the nontacky areas of photosensitive elements of quality comparable to dry nonelectroscopic toners surface coated as taught in U.S. Pat. No. 4,397,941, as well as high optical density, particularly on magenta and cyan toners, together with excellent aging stability with respect to density and background color. However, with some mixing equipment, the toners are coated with the two liquids sequentially, which complicates manufacture, testing and analysis of the toners.
U.S. Pat. No. 4,565,773 to Matrick describes dry nonelectroscopic toner particles surface coated with a single liquid, a nonionic siloxane-oxyalkylene block copolymer. When fresh, these toners have satisfactory cleanup qualities and are nonstaining of the nontacky areas of the photosensitive element of quality comparable to that provided by known dry electroscopic toner particles, such as those surface coated as taught in U.S. Pat. Nos. 4,397,941 and 4,546,072. However, upon aging these toners surface coated with a nonionic siloxane-oxyalkylene block copolymer have poor clean-up properties.
It is desired to provide dry nonelectroscopic toners which can be surface coated with a single compound to provide improved aging stability, in addition to clean-up qualities and nonstaining of nontacky areas of the photosensitive element comparable to known dry nonelectroscopic toners, such as those described above. Preferably, the coating compound is a neat liquid at the application temperature.